1. Field of the Invention
The present invention relates to an automatic running speed control system for automotive vehicles, particularly to an automatic running speed control system for automotive vehicles capable of realizing constant running by a switch operation including overdrive area and also capable of carrying out a fine speed reduction of the car.
2. Description of the Prior Art
Generally, in an automatic running speed control system, when a set switch is operated by a driver in a predetermined car running area such as 40 km/h (24.9 mile/h) to 100 km/h (62.2 mile/h) of running, the car speed at the time of operating the switch, i.e., the time of shifting to the off condition after operating the set switch to on becomes the set car speed when carrying out the automatic constant speed running. After that, a throttle valve of an air intake system of the engine is automatically controlled so as to always coincide the car speed with the set car speed. In this case, when controlling the throttle valve opening, an acutator which is normally a diaphragm type and having a release valve for controlling the communication to open air and a control valve for controlling the negative pressure which occurs in the engine air intake is normally used, whereby the conduction time for the control valve is controlled under the closed condition of the release valve so as to set the operating amount of the throttle valve due to the negative pressure in accordance with the conduction time.
Moreover, in this kind of system, according to the prior art, it is constructed in such a manner that a fine speed reduction can be performed by a switch operating during the automatic constant speed running and when the switch operation is carried out, the throttle valve opening is gradually reduced through the actuator and the car speed is in turn, gradually lowered thereby.
The above-mentioned conventional car includes the overdrive area as a speed changing means, when a find speed reduction operation is performed, however there is a tendency that some difficulties exist in the engine brake because of the gear ratio of less than 1 for the overdrive. Moreover, the response to the engine is not good due to the coming out of the torque converter, thus making the operation of a sufficient and effective fine speed reduction difficult.